It is known for an electric power steering system arranged in response to the steering action of a steering member such as a steering wheel for driving an electric motor in order to transmit the driving force of the electric motor to a steering mechanism for assisting the action of steering that the steering torque exerted on the steering member has to be detected and used for controlling the action of the electric motor to optimize the assisting action and it is therefore needed for detecting the steering torque to employ a torque detecting apparatus which is mounted at a midway on the steering shaft between the steering member and the steering mechanism.
Such a torque detecting apparatus is generally arranged in which when the steering shaft to be examined has been separated into two, first and second, shafts joined to each other coaxially and is loaded with a steering torque by the steering action of the steering member, it creates a relative angle displacement between the first and second shafts from which the steering torque is then calculated. The first and second shafts are joined to each other by a torsion bar at a small diameter of which the torsion spring constant is predetermined for producing the relative angular displacement in proportion to the torque exerted.
The detection of the relative angular displacement between the first and second shafts may be implemented by various known means. One of the known means is a torque detecting apparatus which includes a cylindrical magnet arranged rotatable together with the first shaft and yoke rings arranged rotatable together with the second shaft to detect the relative angular displacement from a change in the magnetic circuit between the cylindrical magnet and the yoke ring (for example, as shown in Patent Citation 1).
As each of the yoke rings arranged rotatable together with the second shaft has a plurality of magnetic pole pawls provided on the circumference of a yoke body at a ring shape thereof to extend in the axial direction, the two yoke rings are fixedly mounted to the second shaft so as to face each other with their magnetic pole pawls aligning alternately along the circumferential direction. The cylindrical magnet arranged rotatable together with the first shaft comprises a set of multiple-pole magnets of which the magnetic poles are aligned along the circumferential direction in the same number as of the magnetic pole pawls of the yoke rings and is fixedly mounted to the first shaft so that their N and S poles correspond to those of the magnetic pole pawls of the yoke rings.
A pair of magnetism collecting rings for collecting the magnetic flux induced by the yoke rings are provided at the outside of the two yoke rings to surround closely the outside of the yoke bodies respectively. The magnetism collecting rings have magnetism collecting projections thereof provided to extend radially and outwardly while a magnetic detector device or magnetic sensor such as a Hall device is disposed between the two opposite magnetism collecting projections of their respective magnetism collecting rings which are spaced by an air gap from each other.
In action, when the first and second shafts receives a steering torque and are varied in the relative angular displacement between them, the positional relationship between the magnetic pole pawls of the yoke rings and the magnetic poles of the cylindrical magnet shifts in opposite directions thus to create a change in the magnetic flux on the yoke rings thus increasing or decreasing the magnetic flux received by the air gap between the two opposite magnetism collecting projections of their respective magnetism collecting rings. This results in a change in the output of the magnetic sensor from which the (steering) torque exerted on the first and second shafts can be calculated.
Since its magnetism collecting rings are located about the yoke rings, the torque detecting apparatus accompanied closely with vehicle mounted loudspeakers including permanent magnets may be affected by the magnetic field induced by the permanent magnets in the loudspeakers thus producing an error in the measurement of detection. It is hence contemplated for eliminating such an error in the measurement of detection derived from the external magnetic fields that the torque detecting apparatus is entirely protected with magnetic shielding members or its housing in which the first and second shafts are installed is coated with a magnetic shielding material.
Patent Citation 1: Japanese Patent Application Laid-Open No. 2003-149062.